Certain N,N'-arylenedimaleimides, particularly N,N'm-phenylenedimaleimides, have long been known as vulcanization adjuvants for elastomers. The orthometa-, and para- isomers of N,N'-arylenedimaleimides are prepared by reacting an arylenediamine and maleic anhydride in the presence of a polar organic solvent to form an N,N'-aryldimaleamic acid precursor, and dehydrating the latter by ring closure with a lower fatty acid anhydride and an alkali metal salt of a lower fatty acid to form the corresponding dimaleimide.
A typical example of this process is the formation of N,N'-m-phenylenedimaleimide. This involves the addition of maleic anhydride to a solution of m-phenylenediamine in dimethylformamide (DMF) and reacting these at a temperature of 35.degree.-40.degree. C. to form N,N'-m-phenylenedimaleamic acid, which is not isolated. The addition of sodium acetate and acetic anhydride follows, with a maximum reaction temperature for ring closure of 55.degree.-60.degree. C. On the completion of this reaction, a large excess of water is added, precipitating the N,N'-m-phenylenedimaleimide which is subsequently filtered, washed with water, and dried at a moderately elevated temperature. This process is outlined below: ##STR1## N,N'-m-phenylenedimaleimide
This process is plagued by low yields (60-65%) and a product isolation process which requires large amounts of water to precipitate the end product. However, the problem posing the greatest economic and ecological concern for this process is the conversion of the necessary expensive and moderately toxic DMF solvent by acetic acid to large amounts of dimethylacetamide and formic acid during the reaction. The consumption of solvent necessitates expensive disposal measures in order to meet standards for environmental quality, since about three to four parts of DMF are consumed and thus discarded per one part of end product. The low solubility of the dimaleamic acid aggravates this problem by requiring large amounts of solvent. Moreover, one must avoid extremes of temperature in the ring-closure step (less than 40.degree. C. or greater than 65.degree. C.) in order to enhance the reaction rate and the solubility of the dimaleamic acid without destroying significant amounts of the dimaleamic acid via side reactions that become prominent at higher temperatures. One desirable product, N,N'-(4-methyl-m-phenylene) dimalemide, cannot be easily isolated when DMF is used as solvent.
Attempts have been made in the past to employ alternative solvent systems. Use of formamide instead of DMF also resulted in the reaction of the solvent with the acetic acid generated, while giving poorer yields and requiring increased amounts of sodium acetate. Processes using different solvents for the two different steps have been successful in the laboratory, but have proven impractical in larger scale production due to the expense and time involved in removing the first solvent before proceeding with ring-closure. An example of such a process is the synthesis of the dimaleamic acid in an aqueous solution, isolating it, and carrying out the conversion to the dimaleimide in a nonaqueous solvent. This latter reaction will not proceed in an aqueous solvent.
The aim of this invention is to provide a synthetic method for maleimides and dimaleimides which will reduce costs and decrease ecological hazards.